Organic friction modifiers have been used in lubricating oil applications for many years. Friction modifiers allow lubricants to achieve friction characteristics necessary for smooth operation of e.g. transmission fluids, tractor fluids, brake fluids, and hydraulic fluids, and also improve fuel economy in engine oils.
The most cost-effective friction modifiers are often C10-C30 organic compounds with a linear or nearly linear non-polar group at one end, and a polar functionality such as a carboxylic acid, a carboxylic acid derivative such as an ester, amide, or salt, an amine, or an alcohol or diol, at the other end. Such friction modifiers function through by forming adsorbed layers on a metal surface, with the polar end attaching to the metal, and the non-polar end sticking out into the lubricant.
In order to be adsorbed from the lubricant onto the metal, friction modifiers must be only marginally soluble in a lubricant. This can cause problems with solubility of the friction modifier in the finished lubricant. In addition, since additive suppliers generally furnish additives to lubricant manufacturers in the form of a mixture of additives, or additive composition, solubility of the friction modifier in the additive composition is also a concern. These problems are exacerbated when the friction modifier is used at high concentrations, or when the friction modifier is a solid at ambient temperatures.
In addition, we have discovered that the presence of co-additives such as high molecular weight dispersants can also decrease the solubility of friction modifiers in lubricating oils and lubricant additive compositions.